It is a rare and alarming quality that bestows an actor with the puppetry skills to have a viewer truly at his mercy. Take French-Algerian actor Tahar Rahim. Such is the sheer mastery of his performances that in any given scene – with the subtle curl of a smirk, caged eyes darting with furious desperation, or devastation writ across his brow – with the flip of a switch, admiration turns to stomach-churning unease, repulsion, or empathy.

During his blistering breakthrough, A Prophet, we see the actor’s face harden before our eyes as six years in prison takes him from naive small-time crook to criminal kingpin. In his seductive turn as globe-trotting serial killer Charles Sobhraj for true crime drama The Serpent, his beguiling warmth and hospitality envelops his victims before a stony stare flicks up like a switchblade in the most brutal of scenes.

Next up, Rahim steps into his extraordinarily tough portrayal of Mohamedou Ould Slahi, a prisoner held at Guantánamo Bay for 14 years in the critically-acclaimed drama The Mauritanian. The film is based on the experiences that the real Slahi chronicled in his shocking 2015 memoir Guantánamo Diary. As can be imagined, it is a visceral performance which delicately flits between hope, despair and elation – a swinging pendulum of emotion that grips the stomach of the viewer throughout the entirety of the film. It is a testimony to Rahim’s dedication to his craft that the actor insisted the torture scenes be as realistic as possible, requesting real shackles during filming, the cells to be kept freezing cold, and that he be waterboarded for real. It would have been “impossible” to have it any other way, he insists when we speak over Zoom. It is an important and altogether star-making role; one that has already earned him his first Golden Globe nomination, and is no doubt about to see him hit household name status.

We caught up with Rahim and discussed the life-changing two-year process of The Mauritanian, how he finally “caught” his portrayal of psychopath Charles Sobhraj in The Serpent, and how – next up – he’d loved to star in a Western…

Hi Tahar, congratulations on The Mauritanian, what drew you to telling the story of Mohamedou Ould Slahi?
When I read the script, I felt sad and angry, and that it was a story that needs to be told. I cried, which is very rare. I just wanted to be part of the people who were doing him justice. It was astonishing to discover Mohamedou and his philosophy, his will to forgive and his ability to put himself in the shoes of others.

You can see from the film, and also from your portrayal, that Mohamedou is this charismatic, engaging and positive person. How was it meeting him knowing everything he had been through?
I first wanted to read the book and do my research so I knew more about what happened there, also about Guantánamo and American justice. Then I met him on Skype because I was shooting The Serpent in Thailand and I was pleasantly surprised to see someone full of life, without resentment, or anger, and I was like ‘wow’.

Did you have any reservations taking on a role that is not only overtly political, but also physically and mentally demanding?
Not at all. You know when I read the title which was Guantánamo Diary at that time, I had a little hesitation. It lasted for less than a minute though. I had gotten used to being offered stereotypical parts from Hollywood, but then I thought ‘no, Kevin [Macdonald] is too clever for that.’ I know him, and he wouldn’t ever tell a story that I don’t want to see. And when I read the book I called him straight away and I said, ‘I’m in, I want to do it.’ Apart from the fact that it’s an important story, as an actor, this part is a gift, there’s so many layers, so many colours, so many challenges.

I read you lost more than twenty pounds to be in the role, and you learned two types of Arabic. How challenging was all of this?
Heavy! It was heavy! But I was excited, I worked a lot. I worked with my Arabic coach and my Hassaniya coach as well – which was the Mauritanian language – and we worked over and over and over, and for the torture scenes. I mean, I had to lose weight to physically match what Mohamedou looked like at that time, and plus putting myself as close as possible to his actual conditions helped me a lot to reach an unexpected emotional state.

I read that you asked for your cell to be kept freezing cold and real shackles to be used. Was it really integral for you to get into the exact mindset by implementing these physical conditions?
Yes, it’s always important. This time in particular, I couldn’t do it otherwise. Plus, when you think about it, why re-create when you can create? I mean, out of respect to Mohamedou, to the audience, to my director, I needed to find the authenticity, otherwise I couldn’t believe in what I was doing. It would have been impossible.

You were talking about how you cried after reading the book. Obviously the role was emotionally demanding and it was a two-year process bringing this film to life, how do you think it changed you as a person, and did it affect your psyche at all?
It was tough. There was a specific moment, a one scene when he starts to hallucinate, right? And he sees his mum in his cell.

Oh god, yes, it’s horrible.
This moment was the last day of shooting. I was exhausted. I couldn’t take it anymore, because we were supposed to shoot the scene two days ago and I felt like I was touching a sort of truth, so I was holding it and they kept on postponing the scene and I was like, ‘I can’t keep it anymore.’ So I went to Kevin and I said, ‘listen man, we’ve got to shoot.’ So we did it, and I almost saw my own mum. It was so strange because I lost my mum and Mohamedou did as well, so we had that in common. I could understand it. But it was strange. So I could only do one take, and then I collapsed.

I read that when you were filming The Serpent, your co-stars even commented that your work ethic is insane. And you’ll always ask for one more take. How do you know for yourself when that magic of the scene has been captured?
It’s inside of yourself. You feel like – it’s strange – but you feel you have it. I felt this one. And sometimes even then, what you feel inside of yourself is not what your director gets. It can happen sometimes. Trying another take when my director is happy, a freestyle one, it’s so good because you don’t feel pressure at all. You feel free to try whatever you want and sometimes accidents happen, and accidents are true.

With The Serpent I read you were fake-tanning and shaving your body every two days. Would you say your acting style veers towards method? That it helps to physically embody your character to help you get into the mindset?
The look helped me a lot. We can say method. But you know, you don’t decide to explore the method. It depends on the movie and the part. And it happened to me with The Serpent and The Mauritanian. But not before actually, not really with other movies. The problem with The Serpent was that I found it so hard to catch him. It took me two weeks to be able to choose, to make a choice between – I don’t know – a coffee or a tea. At some point you get self-confident enough to know what you’ve got to do, and then your director directs you here and there; but in the first two weeks I was lost in a way, so I really needed to focus. So, I created my character from the outside – which I never do. It was his look, the way he talks, and I thought of an animal and I picked the cobra as inspiration.

Previously in all your other roles you must have been able to tap into some form of common ground mentally with the characters, but Sobhraj was this psychopath who lacked any empathy. How did you do it?
Getting in his mindset was very complicated. And as you mentioned I needed to find a connection with my character, and I couldn’t find a connection except for one thing he says in episode three, which is: ‘If I wait for the world to come to me, I’d be waiting still. Everything I ever wanted I had to take it.’ And, I was like yes, this is my story of how I became an actor. I’m from the countryside, a working-class family, no one is related with the business, so I had to go to Paris and take it.

Also congratulations as you’ve been nominated for Best Actor at the Golden Globes for The Mauritanian – what was your first reaction?
I mean I don’t think about it, if you start to think about it and the outcome, you’re not really in the moment, and you don’t take advantage of what is happening to you, and what is happening to me is incredible. But when I knew I got recognised this way, I was very happy.

How did you celebrate?
Ah, you know, it was a half celebration because of Covid. So there were six of us in my friend’s flat, and we played music and had a couple of drinks. But it feels good to be recognised in this kind of way because it brings you more self-confidence as an actor and tells you that you’ve made the right choices as well.

It’s incredibly distressing to hear that Mohamedou was imprisoned for 14 years in total, but then to see the footage of him after laughing, smiling and being so positive… If you were to hope that your viewers take something from the film, what do you hope that is?
A lot of things. His philosophy. To pick forgiveness over hate, or anger. Reminding ourselves of our history is always important; if we don’t talk about what we’ve done, we’re doomed to repeat it, so that is important. And I really hope from the bottom of my heart that Mohamedou will one day be able to travel again, because he can’t [right now]. Each time he asks for an application in Europe they say no. And his family, wife and son, are living in Berlin, and he’d like just to go and live there, so we’ll see.

You’re no stranger to difficult and complex roles that explore uncomfortable ground but deserve a voice regardless. But you’ve also done comedies and arthouse films – is there anything you’d like to try next?
I want to make movies that I want to see. Each time I’m trying to be challenged, to do things that I haven’t done before, to explore different fields in terms of acting. I don’t have a dream role, but I wish I could be in a Western someday, for example. I love the genre.

THE MAURITANIAN is available to stream on Amazon Prime from Thursday 1st April.